The present invention relates to a matching circuit that reduces power reflected from a plasma formed in a substrate processing chamber. The invention is useful in a variety of technologies but is particularly useful in the manufacture of integrated circuits.
Plasma processing is a common step in the manufacture of integrated circuits. Common plasma processing steps include plasma enhanced chemical vapor deposition, reactive ion etching, and sputter etching among others. In such plasma processing steps, precise control of the plasma can be important in order to meet the manufacturing requirements of today's integrated circuits.
Typically a plasma is generated by applying RF energy to a coil or plates of a capacitor (inductive vs. capacitive coupling). To efficiently couple RF energy into a substrate processing chamber matching networks have been used to minimize the energy reflected from a plasma back into the RF generator. The source impedance of an RF generator is constant, typically 50 ohms resistive and zero ohms reactive, while the load of the plasma is transient and variable. The matching network matches the impedance of the load to an RF source from the perspective of the source. Thus, matching networks maximize RF power supplied to the load by minimizing the RF energy reflected from the load.
A variety of matching networks have been developed and successfully used in substrate processing. FIG. 1 is a block diagram of a previously known ac energy delivery system 10. As shown in FIG. 1, energy delivery system 10 includes a matching network 20a coupled by transmission lines 30a-b between an ac power source 40 and a plasma load 50. The matching network is comprised of tuning elements 60a-b that include capacitors, or inductors, or both. The matching network of FIG. 1, having tuning element 60a in parallel with the ac power source and the plasma load and having tuning element 60b in series with the source and load is commonly referred to as an “L network.”
FIGS. 2 and 3 are block diagrams of energy delivery systems 12 and 14 having other previously known matching networks 20b and 20c, respectively. Matching network 20b of FIG. 2 is commonly referred to as a “T network.” T networks typically have one tuning element 60a coupled in parallel with the ac power source 40 and plasma load 50 and have two tuning elements 60c and 60b in series with the ac power source and plasma load. Matching network 20c shown in FIG. 3 is commonly referred to as a “π network.” Typically π networks have two tuning elements 60a and 60d coupled in parallel with ac power source 40 and plasma load 50 while having three tuning elements 60b, 60c and 60e in series with the ac power source and plasma load.
The tuning range of a matching network is a measure of the range of impedance for which disparate load and source impedances can be effectively matched. For example, if the impedance of an ac power source is 50 ohms resistive and a load is 100 ohms resistive and 10 ohms reactive but varies by +/−10 ohms resistive and +/−5 ohms reactive, a matching network tuning range would be sufficiently broad to effectively match these impedances. The tuning range of a matching network is typically related to the number of tuning elements in the network. Thus, a π network typically has a broader tuning range than a T network and a T network typically has a broader tuning range than an L network. However, matching networks having a relatively large number of tuning elements have a relatively higher resistance than matching networks having fewer tuning elements. Thus, total ac energy transfer is typically lower in matching networks with a relatively large number of tuning elements.
Matching networks such as networks 20a-20c shown in FIGS. 1-3 can include tuning elements that are fixed or variable. Variable tuning elements, which include variable capacitors, and/or variable inductors, provide a matching network with continuously adjustable impedance matching. Such continuous adjustability provides the benefit of continuously matching the impedance of an ac power source to a load that has transient and variable impedance. Thus, a controllable amount of energy may be transferred to a load. For example, if the load is a plasma having a transient and variable impedance, by supplying a controllable amount of energy to the plasma through impedance matching, the plasma can be maintained in a relatively stable state.
The cost of variable components is considerably higher than the cost of fixed components. Thus, matching networks that use fixed components are generally less expensive than matching networks that use variable components. Such fixed-element matching networks have limited impedance matching capability, however. Thus, optimal impedance matching is not always achieved with fixed components. To partially overcome the lack of continuous adjustability using fixed components, some previously known matching networks include parallel banks of fixed component tuning elements to provide step wise adjustability. FIG. 4 is a block diagram of an energy delivery system 16 having a step wise adjustable matching network 20d. Step wise adjustability is achieved by switching banks of tuning elements 70a . . . n into or out of connection with ac power source 40. Each bank of tuning elements 70a . . . n can be any of the previous described configurations of tuning elements, “L network”, “T network” or “π network.” While step wise adjustable matching networks provide improved impedance matching capabilities, such matching networks have regions for which optimal energy coupling to a plasma load cannot occur.
Accordingly, it is desirable to develop matching networks that have low cost fixed components while providing improved impedance matching over a broad range of RF wavelengths and high energy transfer.